This invention relates to ink jet systems in which a stream of electrically conductive ink is supplied to a nozzle. The ink is emitted from the nozzle as a stream, but breaks into discrete droplets due to the application of energy to the nozzle using, for example, a piezo-electric device. Near the point where the stream breaks into droplets, an electrode is provided which can selectively charge droplets. This causes them to be deflected onto a substrate to be marked by virtue of deflection electrodes located downstream of the charging electrode.
Such systems are well known in this art and may include drop-on-demand and continuous jet systems. In most cases, such systems use specially formulated inks for quick drying, clear marking and other characteristics which are desired by the user. These devices are temperature sensitive and therefore variation in ambient temperatures, such as in factories where products to be marked are being manufactured, adversely affect printing. For this reason, ink jet printing systems frequently locate the electronics and ink supplies inside a protective cabinet which is located remotely from the point where products are to be marked by the ink drops. The ink jet printhead including the nozzle is located at the point of marking and is connected to the cabinet by a relatively long (ten to thirty feet) umbilical-like tube which supplies both ink and electrical control signals to the printhead assembly.
Under these circumstances, it is somewhat difficult to maintain the ink at the optimum temperatures desired for best printing. For example, many ink jet systems are rated for use in environments within the temperature range of 40.degree. to 110.degree. Fahrenheit. Many inks, however, optimally operate within a temperature range of as little as plus or minus five degrees. For example, an ink formulated for use at 75.degree. F. is desirably maintained between 70.degree. and 80.degree. F. during printing operations.
For this reason, means for controlling the temperature of ink used in ink jet systems are known. Prior efforts to control ink temperature have, however, centered on temperature control within the cabinet housing the principal ink jet components, at a location remote from the actual printing operation. Such prior art systems have principally focused on cooling inks before sending them through the umbilical to the printhead although it is also known to heat the ink at the cabinet. This is a less than effective method because of the length of the umbilical cord and the relatively small volume and low flow rate of ink which is transmitted therethrough. As a result of these factors, temperature control at the cabinet is generally unsatisfactory. Ink is at ambient temperature by the time it reaches the printhead at the end of the umbilical. Accurate temperature control, therefore, cannot be accomplished from the cabinet of such a printing system.
Failure to accurately control ink temperature at the printhead causes a change in flow time (or a change in flight time) of the ink. Most printing systems incorporate detecting mechanisms for measuring flow time and use this value to adjust the composition of the ink, as for example by adding solvent, in an effort to compensate for solvent evaporation. Existing detection systems cannot differentiate between a change in flow time due to solvent loss and changes due to temperature variation. Therefore, such monitoring systems adjust the viscosity of the ink by adding solvent whether the ink requires such addition or not. This can result in an ink composition which is very different from the composition intended, resulting in printing problems and loss of quality.
It is also known in the art to preheat ink at the printhead. Such intentional preheating is used, for example, to reduce the ink viscosity or decrease drying time. Such a concept does not provide positive temperature control at the printhead to maintain the ink within a desired temperature range, for example, plus or minus five degrees of its optimum value.
Accordingly, it is desired to provide a temperature control system which can maintain ink temperature within a predetermined, acceptable range of temperatures in the immediate environment of the printhead.
It is further desired to provide such a temperature control system which has a feedback circuit to monitor ink temperature and the capability to heat or cool the ink as required to maintain the desired temperature range.
It is a further object of the invention to provide such a temperature control system which is located directly in the printhead so that there will be a negligible change in the ink temperature from the time it is adjusted by the temperature control unit and the time it is supplied to the nozzle for printing purposes.
Another object is to provide a miniaturized heat pump for incorporation directly into the printhead to maintain temperature control.